Various methods of bonding substrates are known in the prior art. One bonding method which is used extensively in the manufacture of heat exchanger assemblies is furnace brazing, most commonly by Controlled Atmosphere Brazing (CAB) and Vacuum Brazing (VB). Heat exchanger assemblies are characterized by a plurality of thin aluminum components which are bonded together by brazing. At least some of these components may be comprised of an aluminum brazing sheet having an aluminum or aluminum alloy core layer provided on one or both sides with an aluminum-silicon cladding layer. The cladding layer has a lower melting point than the core layer and melts during brazing to form a filler metal which flows into the voids between the components and solidifies on cooling, thereby bonding the components together.
Because sophisticated rolling mill practices are required to produce aluminum-silicon clad brazing sheet, this composite is more costly than conventional flat rolled sheet and strip. Also, available alloy compositions are limited by mill product standardization, by casting limitations, or by scrap recovery considerations that affect the economy of the overall casting or mill operation.
Conventional brazing alloys can be brazed in the absence of a brazing flux when at least one additional layer of a braze-promoting metal such as nickel, cobalt or iron is applied to the brazing sheet. Such braze-promoting layers are usually applied by electroplating, for example as disclosed in U.S. Pat. No. 4,028,200. If properly applied, the braze-promoter reacts exothermically with the underlying aluminum-silicon cladding alloy, and is believed to disrupt the aluminum oxide layer to permit the underlying aluminum metal to flow together and join. However, there are environmental hazards and liabilities associated with prior art wet electroplating systems for deposition of fluxless braze promoters. Furthermore, there are limitations on the range of material strip or component dimensions which can be electroplated in high volume production. For example, the constraints of fixed size plating cells limit the maximum plateable strip width.
There remains a need for layered products for fluxless brazing of substrates which overcome the problems described above which limit the usefulness of presently known fluxless brazing products.